Depleted uranium, also called DU, is here defined as uranium alloy containing at least 75 weight percent of uranium, such uranium having less U.sup.235 isotope content than is found in the isotopic contents of natural uranium as mined. Likewise, DU oxides are here defined as the oxidation products of DU; such oxidation products may include compounds and elemental residues of th original alloying elements in the DU which was oxidized, and the uranium may be as simple oxides or complex oxides such as sodium uranate or MgU.sub.2 O.sub.6.
DU is becoming widely used in armor-penetrating projectiles. Such projectiles are often fired into sand targets in connection with field tests or personnel training. Because of the presence of low levels of readioctivity from the DU, it is difficult to dispose of the target sands when it periodically becomes necessary to replace them. Specifically, it is customary to store these sands in drums at protected locations where the drum contents cannot be scattered. Such storage is both troublesome and expensive. It would be economically desirable if the radioactive sands could be cleaned enough to be suitable for unprotected disposal.
Current practice in disposal of target sands has sometimes involved screening such sands to remove large pieces of DU plus leaches with nitric acid. However, with several of the alloy compositions used in DU, such nitric-acid leaches can leave insoluble residues which are too radioactive for uncontrolled disposal and which can be explosive. The usual treatments for dissolution of small amounts of such residues (especially where uranium containing much U.sup.235 is involved) have been to add hydrofluoric acid in small amounts to the nitric acid or to fuse complete samples in basic salts from which the uranium can be leached with aqueous solutions such as nitric acid. However, complete dissolution of the target sands would be pointless, and the target sands will react readily with hydrofluoric acid or fused, basic salts. Therefore, new technology is needed for economic and other reasons so that the target sands may be cleaned and their DU content may be recoverd, e.g., as oxides or as uranyl nitrate.
Other prior art has shown (a) that molten nitrates are substantially inert toward sand, (b) that molten nitrates can oxidize uranium alloys, (c) that air oxidation can assist molten nitrates in converting uranium to its hexavalent state, (d) that uranium in its hexavalent state is soluble in aqueous acids such as nitric, and (e) that uranium in its hexavalent state in aqueous solution can be extracted into numerous organic solutions, thereby substantially separating the uranium from the other contents of the aqueous solution.
The combination of properties just described forms that basis of a novel method for the cleanup of target sands so as to achieve both recovery of DU oxide and substantial removal of radioactive contamination from the target sands, thereby easing the waste-management problems. Nonobviousness of the method is demonstrated by the absence of development of suitable cleanup procedures in spite of efforts (e.g., at Eglin Air Force Base) by those versed in and practicing the art of DU waste management. The potential economic value of target-sands cleanup lies in sharp reduction in the number of waste drums which must be protected and monitored. Such protection and monitoring are expensive, and there is increasing difficulty and expense in developing new waste-burial grounds and waste storage areas as the older waste areas become filled.
Therefore, a need still exists for a method of removing DU and DU oxides from target sands.